mRNA for the alpha- and beta-subunits of mouse beta-N-acetylhexosaminidase were detected in tissue sections of epididymis and testis using 3H-labelled antisense RNA probes by hybridization in situ. There were clear differences in the intensity of the signals given by the two tissues with alpha-subunit mRNA being more abundant in testis than epididymis, while the opposite was so with beta-subunit mRNA. Differences in the isoenzyme patterns of the two tissues are likely to be due, at least in part, to differences in transcription by the Hexa and Hexb genes.